The present invention relates to a CATV (cable television) scrambling and descrambling method.
Conventional systems of this type are intended to permit only designated signal receiving units to receive chargeable programs. An example of such a system is shown in FIGS. 1 and 2, of which FIG. 1 shows a scrambling unit and FIG. 2 shows a descrambling unit.
In the scrambling unit, a TVIF modulator 1 converts baseband video and audio signals into a video IF signal and an audio IF signal at intermediate frequencies, and a synchronizing separator circuit 2 separates a synchronizing signal from the video IF signal. The synchronizing signal thus separated is applied to a horizontal and vertical synchronizing compression pulse generating circuit 5 and a timing signal generating circuit 6. In the circuit 5, the output signal of the synchronizing separator circuit 2 is utilized to produce horizontal and vertical synchronizing interval pulses which in turn are used to compress the synchronizing component of the video IF signal, which is applied to a variable gain amplifier 3. The synchronizing signal is converted to a timing signal by the timing signal generating circuit 6. The timing signal is applied to a second variable gain amplifier 4 which receives the audio IF signal to thus superpose the timing signal on the audio IF signal. In the output signals of the variable gain amplifiers 3 and 4, as shown in FIGS. 3A and 3B, the superposed timing signal occurs earlier, by a time T, than the synchronizing part shown in FIG. 3A. The output signals of the amplifiers 3 and 4 are applied to a TV up-converter 7 where they are frequency shifted to a predetermined high frequency channel output.
In the descrambling unit, as shown in FIG. 2, a TV converter 8 receives a predetermined high frequency channel signal. The output of the TV converter 8 is applied to an audio signal detector 9 where the audio signal is separated and the timing signal is reproduced. The timing signal is applied to a return pulse generating circuit 10 to generate level-compressing horizontal and vertical synchronizing interval pulses in the high frequency video signal. As shown in FIG. 3C, the output signal of the circuit 10 lags the timing signal b by an interval of T. The output signal of the circuit 10 is applied to a variable gain amplifier 11 where it is superposed on the TV signal outputted by the TV converter to provide a descrambled signal d, which is transmitted to a television receiver.
In the conventional scrambling and descrambling system as described above, if the time difference T between the synchronizing part of the video signal and the timing signal superposed on the audio signal is known, then a descrambling circuit can be easily constructed. That is, in that case, chargeable programs can easily be received without using an authorized descrambling circuit.